GaN, AlN, InN, which are nitride semiconductors, or materials made of mixed crystals thereof, have a wide band gap, and are used as high output electronic devices or short-wavelength light emitting devices. Among these, as high output devices, technologies are developed in relation to Field effect transistors (FET), more particularly, High Electron Mobility Transistors (HEMT) (see, for example, patent document 1). A HEMT using such a nitride semiconductor is used for high output/high efficiency amplifiers and high power switching devices.
Specifically, a HEMT using a nitride semiconductor includes an AlGaN/GaN (aluminum gallium nitride/gallium nitride) hetero structure formed on a substrate, and the GaN layer is used as an electron transit layer. The substrate is usually made of sapphire, SiC (silicon carbide), GaN (gallium nitride), and Si (silicon).
GaN, which is one kind of a nitride semiconductor, has a high saturated electron speed, a wide band gap, high breakdown voltage, and good electrical characteristics. Furthermore, GaN has a polarity in a (0001) direction parallel to a c axis (wurtzite form). Thus, when an AlGaN/GaN hetero structure is formed, due to lattice distortion caused by the difference in the lattice constant between AlGaN and GaN, piezo polarization is induced, and high-density 2DEG (Two-Dimensional Electron Gas) is generated near the interface in the GaN layer.    Patent document 1: Japanese Laid-Open Patent Publication No. 2002-359256    Patent document 2: Japanese Laid-Open Patent Publication No. 2008-98434
Incidentally, when a nitride semiconductor layer including an AlGaN layer and a GaN layer is caused to epitaxially grow on a substrate made of Si or sapphire by a MOCVD (Metal Organic Chemical Vapor Deposition) method, a warpage is generated in the substrate on which films are formed. This kind of warpage in a substrate is considered to be generated due to stress in the formed semiconductor layer due to the difference in the lattice constants between the material forming the substrate and the material forming the semiconductor layer, and the difference in the thermal expansion coefficients between the material forming the substrate and the material forming the semiconductor layer.
As described above, if a warpage is formed in the substrate when a semiconductor layer is caused to epitaxially grow on the substrate, in subsequent procedures, for example, applying resist and forming an electrode, the following problem arises. That is, the substrate is not properly adsorbed, and it is not possible to convey the substrate. Furthermore, if a warpage is generated in the substrate, when exposure is performed with an exposing device, the pattern fluctuates, and exposure is not performed with a desirable precision. Accordingly, failures are caused in the process of manufacturing a semiconductor device, and the yield of semiconductor devices is reduced.